Crosslinking of Biological Material
Crosslinking of biological molecules is often desired for optimum effectiveness in biomedical applications. For example, collagen, which constitutes the structural framework of biological tissue, has been extensively used for manufacturing bioprostheses and other implanted structures, such as vascular grafts, wherein it provides a good medium for cell infiltration and proliferation. However, biomaterials derived from collagenous tissue must be chemically modified and subsequently sterilized before they can be implanted in humans. The fixation, or crosslinking, of collagenous tissue increases strength and reduces antigenicity and immunogenicity. In one aspect of the prestent invention, crosslinking of a drug-containing biological material with genipin enables the resulting material (“biological substance”) with less antigenicity or immunogenicity, wherein the biological material comprises collagen, gelatin, elastin, chitosan, N, O, carboxylmethyl chitosan (NOCC), and the like that has at least one amino functional group for reaction with genipin.
Collagen sheets are also used as wound dressings, providing the advantages of high permeability to water vapor and rapid wound healing. Disadvantages include low tensile strength and easy degradation of collagen by collagenase. Crosslinking of collagen sheets reduces cleavage by collagenase and improves tensile strength. In one aspect of the prestent invention, a collagen strip derived of crosslinked drug-containing collagen sheets may be used to load on the periphery of a stent as a drug-eluting stent to mitigate restenosis or other abnormality. In a further aspect of the prestent invention, the collagen sheet or collagen strip may be made of solidifiable collagen.
Clinically, biological tissue has been used in manufacturing heart valve prostheses, small-diameter vascular grafts, ligament replacements, and biological patches, among others. However, the biological tissue has to be fixed with a crosslinking or chemically modifying agent and subsequently sterilized before they can be implanted in humans. The fixation of biological tissue or collagen is to reduce antigenicity and immunogenicity and prevent enzymatic degradation. Various crosslinking agents have been used in fixing biological tissue. These crosslinking agents are mostly synthetic chemicals such as formaldehyde, glutaraldehyde, dialdehyde starch, glyceraldehydes, cyanamide, diimides, diisocyanates, dimethyl adipimidate, carbodiimide, and epoxy compound. However, these chemicals are all highly cytotoxic which may impair the biocompatibility of biological tissue. Of these, glutaraldehyde is known to have allergenic properties, causing occupational dermatitis and is cytotoxic at concentrations greater than 10-25 ppm and as low as 3 ppm in tissue culture. It is therefore desirable to provide a crosslinking agent (synonymous to a crosslinking reagent) suitable for use in biomedical applications that is within acceptable cytotoxicity and that forms stable and biocompatible crosslinked products.
An example of a genipin-crosslinked heart valve is reported by Sung et al., a co-inventor of the prestent invention, (Journal of Thoracic and Cardiovascular Surgery vol. 122, pp. 1208-1218, 2001) entitled Reconstruction of the right ventricular outflow tract with a bovine jugular vein graft fixed with a naturally occurring crosslinking agent (genipin) in a canine model, entire contents of which are incorporated herein by reference. Sung et al. herein discloses genipin and its crosslinking ability to a collagen-containing biological tissue heart valve used in an animal implantation study.
To achieve this goal, a naturally occurring crosslinking agent (genipin) has been used to fix biological tissue. The co-pending application Ser. No. 09/297,808 filed Nov. 4, 1997, entitled “Chemical modification of biomedical materials with genipin” and its PCT counterpart, WO 98/19718, are incorporated and cited herein by reference. The cytotoxicity of genipin was previously studied in vitro using 3T3 fibroblasts, indicating that genipin is substantially less cytotoxic than glutaraldehyde (Sung H W et al., J Biomater Sci Polymer Edn 1999; 10:63-78). Additionally, the genotoxicity of genipin was tested in vitro using Chinese hamster ovary (CHO-K1) cells, suggesting that genipin does not cause clastogenic response in CHO-K1 cells (Tsai C C et al., J Biomed Mater Res 2000; 52:58-65), incorporated herein by reference. A biological material (including collagen-containing or chitosan-containing substrate) treated with genipin resulting in acceptable cytotoxicity is a first requirement to biomedical applications.
In a co-pending application by one inventor of the prestent application, U.S. patent application Ser. No. 10/067,130 filed Feb. 4, 2002 entitled Acellular Biological Material Chemically Treated with Genipin, entire contents of which are incorporated herein by reference, discloses an acellular tissue providing a natural microenvironment for host cell migration, in vitro endothelialization, or in vivo endothelialization to accelerate tissue regeneration. The genipin-treated biological biomaterial has reduced antigenicity and immunogenicity.
Restenosis in Angioplasty and Stenting
Atherosclerosis causes a partial blockage of the blood vessels that supply the heart with nutrients. Atherosclerotic blockage of blood vessels often leads to hypertension, ischemic injury, stroke, or myocardial infarction. Typically angioplasty and/or stenting is a remedy for such a disease, however, restenosis does occur in 30-40 percent patients resulting from intimal smooth muscle cell hyperplasia. The underlying cause of the intimal smooth muscle cell hyperplasia is mainly vascular smooth muscle injury and disruption of the endothelial lining.
Vascular injury causing intimal thickening can be from mechanical injuries due to angioplasty and/or stenting. Intimal thickening following balloon catheter injury has been studied in animals as a model for arterial restenosis that occurs in human patients following balloon angioplasty. Injury is followed by a proliferation of the medial smooth muscle cells, after which many of them migrate into the intima through fenestrate in the internal elastic lamina and proliferate to form a neointimal lesion.
Vascular stenosis can be detected and evaluated using angiographic or sonographic imaging techniques and is often treated by percutaneous transluminal coronary angioplasty (balloon catheterization). Within a few months following angioplasty, however, the blood flow is reduced in approximately 30-40 percent of these patients as a result of restenosis caused by a response to mechanical vascular injury suffered during the angioplasty or stenting procedure, as described above.
In an attempt to prevent restenosis or reduce intimal smooth muscle cell proliferation following angioplasty, numerous pharmaceutical agents have been employed clinically, concurrent with or following angioplasty. Most pharmaceutical agents employed in an attempt to prevent or reduce the extent of restenosis have been unsuccessful. The following list identifies several of the agents for which favorable clinical results have been reported: lovastatin; thromboxane A2 synthetase inhibitors such as DP-1904; eicosapentanoic acid; ciprostene (a prostacyclin analog); trapidil (a platelet derived growth factor)]; angiotensin convening enzyme inhibitors; and low molecular weight heparin, entire contents of the above-referred drugs and their therapeutic effects are incorporated herein by reference. It is one aspect of the prestent invention to provide site-specific administration of the pharmaceutical agents disclosed in this invention to the injury site for effective therapy via a genipin-crosslinked collagen-containing or chitosan-containing biological carrier.
Many compounds have been evaluated in a standard animal model. The immunosuppressive agent cyclosporin A has been evaluated and has produced conflicting results. Jonasson reported that cyclosporin A caused an inhibition of the intimal proliferative lesion following arterial balloon catheterization in vivo, but did not inhibit smooth muscle cell proliferation in vitro. It was reported that when de-endothelialized rabbits were treated with cyclosporin A, no significant reduction of intimal proliferation was observed in vivo. Additionally, intimal accumulations of foamy macrophages, together with a number of vacuolated smooth muscle cells in the region adjacent to the internal elastic lamina were observed, indicating that cyclosporin A may modify and enhance lesions that form at the sites of arterial injury.
Morris et al. in U.S. Pat. No. 5,516,781 disclosed Rapamycin (also known as sirolimus), a macrocyclic triene antibiotic produced by Streptomyces hygroscopicus that has been shown to prevent the formation of humoral (IgE-like) antibodies in response to an albumin allergic challenge, inhibit murine T-cell activation, prolong survival time of organ gratis in histoincompatible rodents, and inhibit transplantation rejection in mammals. Rapamycin blocks calcium-dependent, calcium-independent, cytokine-independent and constitutive T and B cell division at the G1-S interface. Rapamycin inhibits gamma-interferon production induced by Il-1 and also inhibits the gamma-interferon induced expression of membrane antigen. Arterial thickening following transplantation, known as CGA, is a limiting factor in graft survival that is caused by a chronic immunological response to the transplanted blood vessels by the transplant recipient's immune system.
Further, Morris et al. in U.S. Pat. No. 5,516,781 claims the invention is distinct from the use of rapamycin for preventing CGA, in that CGA does not involve injury to the recipients' own blood vessels; it is a rejection type response. The disclosed patent '781 is related to vascular injury to native blood vessels. The resulting intimal smooth muscle cell proliferation does not involve the immune system, but is growth factor mediated. For example, arterial intimal thickening after balloon catheter injury is believed to be caused by growth factor (PGDF, bFGF, TGFb, IL-1 and others)-induced smooth muscle cell proliferation and migration. The above-cited U.S. Pat. No. 5,516,781 is incorporated herein by reference.
In the past, polymer or plastic materials have been used as a carrier for depositing a drug or pharmaceutical agent onto the periphery of a stent to treat restenosis. Example is U.S. Pat. No. 6,544,544 to Hunter et al., entire contents of which are incorporated herein by reference. Hunter et al. discloses a method for treating a tumor excision site, comprising administering to a patient a composition comprising paclitaxel, or an analogue or derivative thereof, to the resection margin of a tumor subsequent to excision, such that the local recurrence of cancer and the formation of new blood vessels at the site is inhibited. The composition further comprises a polymer, wherein the polymer may comprise poly(caprolactone), poly(lactic acid), poly(ethylene-vinyl acetate), and poly(lactic-co-glycolic)acid.
In another example, Biocompatibles PC (phosphorylcholine by Biocompatibles, London, England) has been added as a drug carrier or surface modifier for treating tissue injury due to angioplasty and/or stenting. The technique comprises a hydrophobic component that aids in the initial adhesion and film-formation of the polymer onto the stainless steel stent substrate, and other groups allow cross-linking both within the polymer and with the stent surface to achieve firm anchorage. The coating is thus tenaciously adhered to the stent and can survive balloon expansion without damage. A therapeutic drug can be loaded within the coated substrate, such as phosphorylcholine.
Drugs are usually loaded, admixed or entrapped physically within the polymer framework for slow drug release. The plastic polymer which is suitable as a drug carrier may not be biocompatible, whereas some biocompatible plastic polymer may not be able to contain a specific drug and release drug in an effective timely amount for effective therapy. Therefore, there is a clinical need to have a biocompatible drug carrier that releases an effective quantity of drug over a period of time for prolonged therapeutic effects.
U.S. Pat. No. 5,085,629 issued on Feb. 4, 1992, entire contents of which are incorporated herein by reference, discloses a biodegradable, biocompatible, resorbable infusion stent comprising a terpolymer of: (a) L(−)lactide, (b) glycolide, and (c) epsilon-caprolactone. This invention includes a method for treating ureter obstructions or impairments by utilizing a biodegradable, biocompatible, resorbable infusion stent, and a method for controlling the speed of resorption of the stent. A ureter stent that is made of a biodegradable and biocompatible material would assure its safe and innocuous disappearance without the need for a second surgical procedure for its removal after it has completed its function.
U.S. Pat. No. 5,464,450 issued on Nov. 7, 1995, entire contents of which are incorporated herein by reference, discloses a stent made of biodegradable material including a drug that is released at a rate controlled by the rate of degradation of the biodegradable material. The stent includes a main body of a generally tubular shape. The main body may further include a plurality of apertures extending therethrough and a slot defined by opposing edges which permits insertion and positioning of the stent.
U.S. Pat. No. 6,200,335 issued on Mar. 13, 2001 and No. 6,632,242 issued on Oct. 14, 2003, entire contents of which are incorporated herein by reference, discloses a stent for a vessel inserted in use into the vessel of a living body including a tubular member constituting a passageway from one end to its opposite end. The tubular member includes a main mid portion and low tenacity portions formed integrally with both ends of the main mid portion. The low tenacity portions are lower in tenacity than the main mid portion. These low tenacity portions are formed so as to have the Young's modulus approximate to that of the vessel of the living body in which is inserted the stent, so that, when the stent is inserted into the vessel, it is possible to prevent stress concentrated portions from being produced in the vessel.
In accordance with the prestent invention there is provided genipin treated collagen-containing or chitosan-containing biological material loaded with at least one drug for implant and other surgical applications which have shown to exhibit many of the desired characteristics important for optimal therapeutic function. In particular, the crosslinked collagen-drug compound with drug slow release capability may be suitable as anti restenosis agent in treating atherosclerosis and other therapeutic applications.